The Scottish Parliament could gain more power over tax if voters reject independence, t he Prime Minister has said.

A No vote on September 18 is "not the end of line" for devolution, David Cameron told the Scottish Conservative party conference in Edinburgh.

He stopped short of spelling out any detail on what those powers could be, but insisted that he and Scottish party leader Ruth Davidson want devolution to "work better".

Mr Cameron said: "Let me be absolutely clear: a vote for 'no' is not a vote for 'no change'.

"We are committed to making devolution work better still.

"Not because we want to give Alex Salmond a consolation prize if Scotland votes No, but because it's the right thing to do.

"Giving the Scottish Parliament greater responsibility for raising more of the money it spends - that's what Ruth believes, and I believe it too.

"So here's the re-cap: vote yes, that is total separation.

"Vote no, that can mean further devolution - more power to the Scottish people and their parliament, but with the crucial insurance policy that comes with being part of the UK."

The pro-independence Yes Scotland campaign group said the promise is a sham.

Chief executive Blair Jenkins said: "The trouble is that we have heard it all before and it is clear from what the Prime Minister said today in his speech that he cannot and will not guarantee more powers for the Scottish Parliament.

"With Labour also hopelessly split on more powers, the message to take from all of this is very clear: that the only way the people of Scotland can be guaranteed to get the powers they want and the country needs is to vote Yes on September 18."

In his 20-minute speech, the Prime Minister insisted there will be "no going back" if voters say Yes to independence.

He praised the "spirit" of the party in Scotland, which has just one MP at Westminster.

And he drew on recent interventions from major businesses, such as BP and Shell, in the referendum debate.

He also vowed to tackle accusations of "scaremongering" over the country's future, saying he wants to "take that myth apart".

"Six months from now the day will dawn, the polling booths will open, the voters will come out and the people of Scotland will decide - stay or go, stick with the UK or walk away," he said.

"If the Scottish people vote Yes in September, then Scotland will become an independent country, there will be no going back, no second chances."

He added: "We face a monumental battle to keep our United Kingdom together."

An opinion poll this week put support for independence at its highest level for more than six months. The Survation poll suggests 39.3% will vote for independence, compared with 47.6% who said they want Scotland to remain part of the UK.

Mr Cameron's opponents argue that Mr Cameron's government does not reflect Scottish "values".

But he told the conference: "Our values are Scotland's values. We've got to make this case fearlessly, passionately, and that is exactly what our brilliant leader Ruth is doing today."

He repeated the warning that the UK Government would not agree to a formal currency union to allow an independent Scotland to keep sterling.

"There are a few myths doing the rounds," he said.

"There's the myth that any talk about the consequences of separation is all bluff and bluster, or even bullying. Warnings on the currency. Warnings on the EU.

"The nationalists say this is a big political conspiracy from south of the border, just ignore it. But that is wrong and, frankly, irresponsible."

He defended interventions from the president of the European Commission, BP, Shell, RBS and Barclays.

"These are not political puppets, they are serious, non-partisan figures," he said.

"So the idea that these are empty warnings and political scaremongering is a myth, and we owe it to the people of Scotland to take that myth apart."

The Prime Minister went on to emphasise the UK's shared history, values, identity and culture, touching on events such as the First World War, the Olympics and the Commonwealth Games.

Remaining part of the UK would also provide many benefits for industry, he told delegates.

Sectors such as oil and gas and defence are ''better off'' as part of the UK, he said.

Having a ''place around the top table'' at European and international organisations also ensures the best trade deals, he told delegates.

''And I always make sure personally that Scottish-based companies are there with me at the top table.

''Like Aggreko, headquartered in Glasgow. I've taken them on trade missions to Brazil, India, South East Asia, shaking hands and doing deals that mean more work and more wealth right here in Scotland.

''Our place in the world matters - and the fact is we matter more as a United Kingdom.''

The nations within the UK are ''a family'', the Prime Minister concluded.

''We're there for each other,'' he said.

''When you look around the world, so often, tragically, neighbourliness has been lost and replaced by wars and feuds.

''We don't do that. We don't slam the doors and turn compatriots into foreigners. We work together.''

Deputy First Minister Nicola Sturgeon said: "David Cameron has today torn up any prospect of a meaningful offer of further powers to Scotland, confirming that a No vote offers no firm proposals.

"In a speech that contained half-hearted, vague suggestions of what might happen, Cameron's reluctance to commit to anything spoke volumes. Indeed, the only reason the Tories are even talking about more powers is to attempt to bribe the people of Scotland into voting No - but it is unravelling fast.

"The No camp is clearly getting increasingly rattled as recent polls show more and more people are moving to Yes.

"But for all his talk of fighting, Cameron is still running scared on independence. Despite claiming numerous times in today's speech that he would fight for a No vote, he consistently refuses to debate the First Minister."

The Tories have "no mandate" in Scotland, she said.

"It is time to put an end to Tory governments in Westminster we didn't elect imposing policies that inflict pain on people across Scotland. Only a Yes vote in September can ensure this happens," she added.